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    SubjectRe: [PATCH] locking/spinlock/debug: snapshot lock fields
    On Mon, Oct 23, 2017 at 01:29:10PM +0100, Mark Rutland wrote:
    > Currently, the lock debug code doesn't use {READ,WRITE}_ONCE() to access
    > lock fields, and thus may observe torn values given a suitably unhelpful
    > compiler. These could result in false positives and/or false negatives
    > for some sanity checks.
    >
    > Further, as we don't snapshot the values of various fields, these might
    > change between the time they are sanity checked and the time they are
    > logged, making debugging difficult.
    >
    > This patch ensures that lock fields are accessed with
    > {READ,WRITE}_ONCE(), and uses a snapshot of the lock to ensure that
    > logged values are the same as those used for the sanity checks.
    >
    > Signed-off-by: Mark Rutland <mark.rutland@arm.com>
    > Cc: Peter Zijlstra <peterz@infradead.org>
    > Cc: Ingo Molnar <mingo@redhat.com>

    ACK, I'll try and remember to look at this if there's anything further
    we can remove from this file.

    > +static inline raw_spinlock_t
    > +debug_spin_lock_snapshot(raw_spinlock_t *lockp)
    > +{
    > + raw_spinlock_t lock;
    > +
    > + lock.raw_lock = READ_ONCE(lockp->raw_lock);

    I think you want to make that an smp_load_acquire(), such that we
    preserve the release-acquire relation for the lock.

    Since this happens _before_ the regular lock, but debug_spin_unlock()
    also happens before the regular unlock. So the unlock release is still
    placed right. But we've messed up our acquire.

    > + lock.magic = READ_ONCE(lockp->magic);
    > + lock.owner_cpu = READ_ONCE(lockp->owner_cpu);
    > + lock.owner = READ_ONCE(lockp->owner);
    > +
    > + return lock;
    > +}

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